One of the key parameters which must be controlled in the manufacture of cord reinforced tire fabric is its weight per unit area (known as "basis weight"). If basis weight is not determined during the manufacturing process, and adjustments to the process not made to correct deviations from the desired basis weight, large amounts of out of specification material could be made before the process can be corrected. Since steel reinforcement cords are most commonly used in the manufacture of tire fabric, the following specification describes the invention in connection with steel wire reinforced tire fabric. It should be understood, however, that the principles of the invention are applicable to fabric using other types of reinforcing materials.
Continuous determination of basis weight per se is not new. One method which has been used involves measuring the attenuation of radiation such as X-rays or beta radiation as it passes through the material being manufactured. It has long been known that radiation passing through a substance is attenuated in accordance with an exponential function variously known as Beer's Law, Lambert's Law, or Bouguer's Law. As applied to a homogeneous material, a continuous determination of the basis weight can therefore easily be made from a knowledge of the "extinction coefficient" for the material and the attenuation ratio observed when the product is irradiated with an appropriate type of radiation. The "extinction coefficient" is a constant which accounts for both absorption and reflection effects.
Unfortunately, cord reinforced tire fabric is not homogeneous, and basis weight can change due either to changes in the size or spacing of the reinforcing cords, or the thickness of the rubber. This leads to ambiguities in applying the exponential decay equation so that the desired determination cannot be made. To account for the additional variables introduced by the wire cords, it has been suggested that two sources of radiation be used which are absorbed differently by the rubber and the cord. Such a system is described by Bossen in U.S. Pat. No. 3,889,121. The calculations necessary to apply this method, however, are relatively complicated and a number of approximations are required to arrive at a result. A more direct means of determining basis weight is accordingly desirable.